Cooled photocathode structure

ABSTRACT

A photocathode for an image intensifier tube includes a faceplate, a glass plate disposed opposite the faceplate, and a span having one end attached to the glass plate and the other end attached to the faceplate for forming a sealed chamber between the faceplate and the glass plate. A semiconductor layer is bonded to a surface of the glass plate, where the surface is disposed outside of the sealed chamber. The semiconductor layer forms a photocathode. A thermal electric cooler (TEC) is disposed inside the sealed chamber for cooling the photocathode. The faceplate is formed from sapphire material. The glass plate is formed from high conductivity glass. The span is formed from either high conductivity glass or low conductivity glass. The faceplate and the glass plate form a path for light to impinge upon the semiconductor layer, and the photocathode of the semiconductor layer is configured to convert the light into electrons for emission toward an electron gain device.

FIELD OF THE INVENTION

The present invention relates, in general, to image intensifier tubesand, more specifically, to a photocathode structure subjected tocooling.

BACKGROUND OF THE INVENTION

Night vision systems are used in a wide variety of military, industrialand residential applications to enable sight in a dark environment. Forexample, night vision systems are utilized by military aviators duringnighttime flights. Security cameras use night vision systems to monitordark areas and medical instruments use night vision systems to alleviateconditions such as retinitis pigmentosis (night blindness).

Image intensifier devices are employed in night vision systems toconvert a dark environment to an environment perceivable by a viewer.More specifically, the image intensifier device within the night visionsystem collects tiny amounts of light in a dark environment, includingthe lower portion of the infrared light spectrum present in theenvironment, which may be imperceptible to the human eye. The deviceamplifies the light so that the human eye can perceive the image. Thelight output from the image intensifier device can either be supplied toa camera, external monitor or directly to the eyes of a viewer. Theimage intensifier device is commonly employed in vision goggles that areworn on a user's head for transmission of the light output directly tothe viewer. Accordingly, since the goggles are worn on the head, theyare desirably compact and light weight for purposes of comfort andusability.

Image intensifier devices include three basic components mounted withina housing, i.e. a photocathode (commonly called a cathode), amicrochannel plate (MCP), and a phosphor screen (commonly called ascreen, fiber-optic or anode). The photocathode detects a light imageand converts the light image into a corresponding electron pattern. TheMCP amplifies the electron pattern and the phosphor screen transformsthe amplified electron pattern back to an enhanced light image.

Referring to FIG. 1, a current state of the art Generation III (GEN III)image intensifier tube 10 is shown. Examples of the use of such a GENIII image intensifier tube in the prior art are exemplified in U.S. Pat.No. 5,029,963 to Naselli, et al., entitled REPLACEMENT DEVICE FOR ADRIVER'S VIEWER and U.S. Pat. No. 5,084,780 to Phillips, entitledTELESCOPIC SIGHT FOR DAYLIGHT VIEWING. The GEN III image intensifiertube 10 shown, and in both cited references, is of the type currentlymanufactured by ITT Corporation, the assignee herein. In intensifiertube 10 shown in FIG. 1, infrared energy impinges upon photocathode 12.The photocathode 12 is comprised of glass faceplate 14 coated on oneside with antireflection layer 16, a gallium aluminum arsenide (GaAlAs)window layer 17 and gallium arsenide (GaAs) active layer 18. Infraredenergy is absorbed in GaAs active layer 18, thereby resulting in thegeneration of electron/hole pairs. The produced electrons are thenemitted into vacuum housing 22 through a negative electron affinity(NEA) coating 20 present on GaAs active layer 18.

A microchannel plate (MCP) 24 is positioned within vacuum housing 22,adjacent NEA coating 20 of photocathode 12. Conventionally, MCP 24 ismade of glass having a conductive input surface 26 and a conductiveoutput surface 28. Once electrons exit photocathode 12, the electronsare accelerated toward input surface 26 of MCP 24 by a difference inpotential between input surface 26 and photocathode 12 of approximately300 to 900 volts. As the electrons bombard input surface 26 of MCP 24,secondary electrons are generated within MCP 24. The MCP 24 may generateseveral hundred electrons for each electron entering input surface 26.The MCP 24 is subjected to a difference in potential between inputsurface 26 and output surface 28, which is typically about 1100 volts,whereby the potential difference enables electron multiplication.

As the multiplied electrons exit MCP 24, the electrons are acceleratedthrough vacuum housing 22 toward phosphor screen 30 by a difference inpotential between phosphor screen 30 and output surface 28 ofapproximately 4200 volts. As is the electrons impinge upon phosphorscreen 30, many photons are produced per electron. The photons createthe output image for image intensifier tube 10 on the output surface ofoptical inverter element 31.

FIG. 2 is a schematic representation of image intensifier tube 41. Thetube includes photocathode 54, microchannel plate (MCP) 53 and imagingsensor 56. Imaging sensor 56 can be any type of solid-state imagingsensor, such as a CCD device, or a CMOS imaging sensor.

Photocathode 54 can be, but is not limited to, a material such as GaAs,Bialkali, InGaAs, and the like. Photocathode 54 includes input side 54 aand output side 54 b. MCP 53 has a plurality of channels 52 formedbetween an input surface and an output surface.

An electric biasing circuit 44 provides a biasing current to imageintensifier tube 41. Electric biasing circuit 44 includes a firstelectrical connection 42 and a second electrical connection 43. Firstelectrical connection 42 provides a biasing voltage between photocathode54 and MCP 53. Second electrical connection 43 applies a biasing voltagebetween MCP 53 and imaging sensor 56. In this configuration,photocathode 54, MCP 53, and imaging sensor 56 are maintained in avacuum body or envelope 61 as a single unit, in close physical proximityto each other.

Still referring to FIG. 2, in operation, light 58, 59 from an image 57enters image intensifier tube 41 through input side 54 a of photocathode54. Photocathode 54 changes the entering light into electrons 48, whichare output from output side 54 b of photocathode 54. Electrons 48exiting photocathode 54 enter channels 52 of MCP 53. Secondary electronsare generated within the plurality of channels 52 of MCP 53. The MCP 53may generate several hundred electrons in each of channels 52 for eachelectron entering through the input surface. Thus, the number ofelectrons 47 exiting channels 52 is significantly greater than thenumber of electrons 48 that entered channels 52. The intensified numberof electrons 47 exit channels 52 and strike the electron receivingsurface of imaging device 56. The imaging device transforms theelectrons into a light image which may be stored in memory or viewed ondisplay 46.

SUMMARY OF THE INVENTION

To meet this and other needs, and in view of its purposes, the presentinvention provides a photocathode for an image intensifier tubeincluding a faceplate, a glass plate disposed opposite the faceplate,and a span having one end attached to the glass plate and the other endattached to the faceplate, for forming a sealed chamber between thefaceplate and the glass plate. A semiconductor layer is bonded to asurface of the glass plate, where the surface is disposed outside of thesealed chamber. The semiconductor layer forms a photocathode. A thermalelectric cooler (TEC) is disposed inside the sealed chamber for coolingthe photocathode. The faceplate is an annular structure; the glass plateis an annular structure, and the span is an annular bracket extendingbetween the glass plate and the faceplate for providing a separationdistance between the faceplate and the glass plate. The faceplate isformed from a sapphire material, or other optically transparent materialof high thermal conductivity. The glass plate is formed from highconductivity glass. The span is formed from either high conductivityglass or low conductivity glass. Preferably, the span is formed from lowconductivity glass or other low conductivity material.

The faceplate and the glass plate form a path for light to impinge uponthe semiconductor layer, and the photocathode of the semiconductor layeris configured to convert the light into electrons for emission toward anelectron gain device. The electron gain device is a microchannel plate(MCP).

At least one cantilever bracket is attached to the glass plate at oneend, and forms a seat for the annular TEC at another end. The at leastone cantilever bracket is formed of copper material to provide thermalconductivity between the TEC and the glass plate. The seat includes anindentation formed in the at least one cantilever bracket for receivingthe annular TEC. The at least one cantilever bracket is bonded at an endto the glass plate. Standoffs are formed on top of the glass plate forproviding a separation distance between the glass plate and the opposingfaceplate.

Another embodiment of the present invention is a photocathode structurehaving a sealed chamber formed by walls, a bottom wall providing anexterior surface to the sealed chamber, a photocathode layer disposed onthe exterior surface, and a TEC disposed within the sealed chamber forcooling the photocathode layer. The TEC is in thermal contact with thephotocathode layer by way of high conductivity material. The highconductivity material includes glass and at least one copper bracketattached to the glass.

Yet another embodiment of the present invention is an image intensifiertube including a photocathode structure, an electron sensing device, andan electron gain device disposed between the electron sensing device andthe photocathode structure. The photocathode structure includes: asealed chamber formed by walls, a bottom wall providing an exteriorsurface to the sealed chamber, a photocathode layer disposed on theexterior surface, and a TEC disposed within the sealed chamber forcooling the photocathode layer. The TEC is in thermal contact with thephotocathode layer by way of a high conductivity material, whichincludes glass and at least one copper bracket attached to the glass.

It is understood that the foregoing general description and thefollowing detailed description are exemplary, but are not restrictive,of the invention.

BRIEF DESCRIPTION OF THE FIGURES

The invention may be understood from the following detailed descriptionwhen read in connection with the following figures:

FIG. 1 is a cross-sectional diagram of a conventional image intensifiertube.

FIG. 2 is a functional block diagram of a conventional image intensifiersystem.

FIG. 3 is a cross-sectional diagram of a first set of components used inassembling a photocathode structure, in accordance with an embodiment ofthe present invention.

FIG. 4 is a cross-sectional diagram of a second set of components usedfor assembling a photocathode structure, in accordance with anembodiment of the present invention.

FIG. 5 is a cross-sectional diagram of an assembled photocathodestructure, using the sets of components shown in FIGS. 3 and 4, inaccordance with an embodiment of the present invention.

FIGS. 6A, 6B and 6C are cross-sectional diagrams and perspectivediagrams, respectively, showing portions of an assembled photocathodestructure, in accordance with an embodiment of the present invention.

FIG. 7 is a plot of wafer temperature versus TEC power, showingperformance results of using an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a photocathode structure that is cooledin temperature to reduce generation of dark currents. It is known that aphotocathode generates dark currents, when its temperature increasesduring operation in an image intensifier tube or in a solid state imageintensifier. The dark currents of the photocathode is temperaturedependent. Lowering the temperature is one method of reducing darkcurrents.

Lowering the temperature, however, requires electrical power, whoseusage is preferably minimized, especially during operation of a nightvision goggle device. In conventional photocathodes (such as shown inFIG. 1), the entire image intensifier system is cooled, by immersing thedevice in an exterior tube. The exterior tube results in an inefficientusage of electrical power, because a large mass is required to betemperature cooled. For example, the tube body, the MCP and thephoto-anode structure are unnecessarily cooled.

As will be explained, the present invention advantageously concentrateson cooling primarily only the photocathode structure. The presentinvention advantageously uses a vacuum formed between the photocathodestructure and the MCP to obtain a high thermal resistance, so that theamount of heat re-entering the photocathode structure is reduced. Thepresent invention also reduces the amount of material comprising thephotocathode structure, in order to reduce the number of paths forre-entrant heat flowing into the photocathode structure. Furthermore,the present invention replaces the reduced amount of material comprisingthe photocathode with a vacuum, which forms a high thermal resistance.

Referring now to FIGS. 3, 4 and 5, there is shown a cooled photocathodestructure, in accordance with an embodiment of the present invention.FIGS. 3 and 4 show two separate sets of components of the photocathodestructure and FIG. 5 shows an integrated and assembled photocathodestructure.

Referring first to FIG. 3, there is shown a first set of components of aphotocathode structure, generally designated as 62. The first set ofcomponents is comprised of faceplate 63 and a thermal electric cooler(TEC) 64. The faceplate 63 may be formed from sapphire material, forexample, and may have an annular cross-section. The top annular surfaceof faceplate 63 is designated as 63A and the bottom annular surface isdesignated as 63B. It will be appreciated that faceplate 63 may beformed of any material having a high thermal conductivity (which, forexample, may be greater than or equal to 33 W/m/k) and of any materialproviding a transparent window for light passing from top surface 63A tobottom surface 63B.

As shown in cross-section in FIG. 3, TEC 64 forms an annular ring. Itwill be appreciated, however, that TEC 64 may be one or more thermalcoolers soldered or fastened to bottom surface 63B of faceplate 63, anddoes not need to be annular in shape. The one or more TECs 64 may beattached directly to the bottom surface of faceplate 63 using only oneelectrically insulating annular ceramic ring (not shown).

The faceplate 63 may include two contact ports for TEC power (not shown)and two contact ports for a thermistor (not shown). The thermistor maybe used to control the on/off operation of the one or more TECs. Thecontact ports may be formed by drilling into faceplate 63. The contactports may be formed by a recess in the bottom surface of faceplate 63,as shown by recess 65 in the faceplate. Of course, for an annular TEC,recess 65 may also be annular to completely receive the TEC. An indiumsealant may be used for sealing any openings in recessed section 65between the TEC and the faceplate. A high temperature solder materialmay also be used for assembling the TEC (one or more) with thefaceplate.

It will be appreciated that a non-evaporable getter may be placed on thebottom surface of faceplate 63.

Referring next to FIG. 4, there is shown a second set of components of aphotocathode structure, generally designated as 66. The second set ofcomponents is comprised of glass plate 67, span 71, one or morecantilevered brackets 69, 70, and semiconductor layer 72.

The span 71 and glass plate 67 may be formed from one type of glass orfrom two types of glass. As shown in FIG. 4, glass plate 67 is formed asa glass disk using high conductivity glass and span 71 is formed as an“L” shape using low conductivity glass. The glass plate 67 is bonded tospan 71 forming a single “U” shape, when viewed in cross-section. Asanother embodiment, glass plate 67 and span 71 may be formed from onetype of glass having high or low thermal conductivity.

As an example, the high conductivity glass may be BK7 having a thermalconductivity of 1.3 W/m/k. The low conductivity glass may have a thermalconductivity of 0.3 W/m/k. It is important, of course, that glass plate67 be made from glass or other material that provides a transparentwindow for light to pass through the glass and impinge on semiconductorlayer 72, the latter converting the light into electrons.

The semiconductor layer 72 is bonded to glass plate 67 for providing thephotocathode transformation of light (photons) into electrons. Theelectrons, of course, are then provided as an input to an MCP (such asMCP 53 shown in FIG. 2). The semiconductor layer may include an activelayer such as gallium arsenide (GaAs) and additional layers, such as anantireflection layer, a window layer of gallium aluminum arsenide(GaAlAs) and a negative electron affinity (NEA) coating disposed on theGaAs active layer (as described with respect to FIG. 1).

It will be appreciated that after forming glass plate 67 and span 71,the formed glass may be ground and polished. The semiconductor layer 72is then bonded to glass plate 67. Next, in a possible fabricationsequence, the surface of glass plate 67, which is opposite tosemiconductor layer 72 may be further ground and polished. Thecantilevered brackets (one or more) may be finally attached to glassplate 67.

As shown in FIG. 4, cantilevered brackets 69, 70 are bonded to the enddisk surface of glass plate 67. The bonding may be performed using fritor solder, for example. The cantilevered brackets may be formed of anyconductive material having high thermal conductivity, such as copper.The cantilevered brackets may be formed as separate sections, as bestshown in FIG. 6B, and attached to the disk surface of glass plate 67 byway of a ring, as shown in FIG. 4 designated as 75. The ring 75 may beformed of materials identical to cantilevered brackets 69, 70. It willbe understood that ring 75 and cantilevered brackets 69, 70 may be asingle piece of copper, for example.

If made from a deformable material, such as copper, cantileveredbrackets 69, 70 may be notched or recessed at their end portions toreceive, hold or lock TEC 64, as shown in FIG. 5.

The final assembly of the first and second sets of components 62 and 66into an integrated photocathode structure is shown in FIG. 5, where theintegrated photocathode structure is designated as 80. In preparationfor assembly, first set of components 62 (FIG. 3) and second set ofcomponents 66 (FIG. 4) may be subjected separately to a UHV (ultra-highvacuum) process. The first set of components 62 may undergo reducedtemperature processing, whereas the second set of components 66 may besubjected to processing in a full temperature range. The reverse,however, may also be true.

The first and second sets of components may be press fitted during theUHV process using an indium bond to form a sealed evacuated chamber. Theindium bond is designated as 81 and the sealed chamber is designated as76, as shown in FIG. 5. Two or more standoffs 68A, 68B may be providedon top of the disk end of glass plate 67 for supporting faceplate 63.

The cantilevered brackets 69, 70 provide support for TEC 64, as shown inFIG. 5. Although not shown, it will be appreciated that the cantileveredbrackets may be notched or recessed to receive and hold TEC 64 inposition. A bond may not be necessary to lock TEC 64 to cantileveredbrackets 69, 70. During the sealing process of first and second sets ofcomponents 62 and 66, the cantilevered brackets may flex and takepressure away from TEC 64. The flexing is very noticeable, when thecantilevered brackets and ring 75 are formed from a single piece ofcopper.

Referring next to FIGS. 6A, 6B and 6C, there is shown an assembledphotocathode structure 80. FIG. 6A is similar to FIG. 5, except that thephotocathode structure is shown up-side down. FIG. 6B is a perspectiveview of photocathode structure 80, with TEC 64 and span 71 (FIG. 6A) notshown. FIG. 6C is a cut-away view of photocathode structure 80, with TEC64 not shown.

FIG. 7 is a plot of wafer temperature)(C.°) versus TEC power (W). Thetwo solid curves having the legend of “BK-7 spacer” implies that glassplate 67 and span 71 are formed from a single high thermally conductivematerial, such as BK-7. The two dashed curves having the legend of “lowK spacer” implies that glass plate 67 is formed from a high thermallyconductive material and span 71 is formed from a low thermallyconductive material. The curves shown in FIG. 7 are results ofsimulation taken at two different ambient temperatures (23° C. and 50°C.). It will be appreciated that the “low K spacer” (2 materials)provides a lower temperature than the “BK-7 spacer” for a fixed TECpower usage.

Accordingly, the present invention provides a low power method ofcooling the photocathode by incorporating the TECs into a vacuumenvironment, such as chamber 76. The vacuum chamber 76 is separate fromphotocathode layer 72, in order to prevent poisoning of the photocathodesurface, because the TEC cannot be processed at a high temperature.

Some penalty is paid by the present invention, due to an increaseddiameter of the cathode, which may be traded off between power usageversus size. In general terms, photocathode structure 80 may be sizedfor insertion into housing 22 of image intensifier tube 10 shown inFIG. 1. Of course, photocathode structure 12 is replaced by photocathodestructure 80 of the present invention.

It will be observed that the vacuum chamber of photocathode structure 80is separate from the vacuum chamber of housing 22, in which thephotocathode layer, MCP 24 and the input surface of anode 31 reside.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

1. A photocathode for an image intensifier tube comprising a faceplate,a glass plate disposed opposite the faceplate, a span having one endattached to the glass plate and the other end attached to the faceplate,for forming a sealed chamber between the faceplate and the glass plate,and a semiconductor layer bonded to a surface of the glass plate, thesemiconductor layer disposed outside of the sealed chamber, wherein thesemiconductor layer transforms light into electrons, a thermal electriccooler (TEC) is disposed completely inside the sealed chamber forcooling the semiconductor layer, at least one cantilever bracket isattached to the glass plate at one end, and the cantilever bracket formsa seat for holding the TEC at another end, the cantilever bracket andthe span each extend away from the glass plate and do not touch eachother, and the cantilever bracket provides direct thermal conductionbetween the TEC and the glass plate.
 2. The photocathode of claim 1wherein the faceplate is an annular structure, the glass plate is anannular structure, and the span is an annular bracket extending betweenthe glass plate and the faceplate for providing a separation distancebetween the faceplate and the glass plate.
 3. The photocathode of claim1 wherein the faceplate is formed from sapphire material.
 4. Thephotocathode of claim 1 wherein the faceplate and the glass plate form apath for light to impinge upon the semiconductor layer, and thesemiconductor layer is configured to convert the light into electronsfor emission toward an electron gain device.
 5. The photocathode ofclaim 4 wherein the electron gain device is a microchannel plate (MCP).6. The photocathode of claim 1 wherein the at least one cantileverbracket is formed of copper material to provide thermal conductivitybetween the TEC and the glass plate.
 7. The photocathode of claim 1wherein the seat includes an indentation formed in the at least onecantilever bracket for receiving the annular TEC.
 8. The photocathode ofclaim 1 wherein the at least one cantilever bracket is bonded at an endto the glass plate.
 9. The photocathode of claim 1 wherein the sealedchamber is a vacuum.
 10. The photocathode of claim 1 including standoffsformed on top of the glass plate for providing a separation distancebetween the glass plate and the opposing faceplate.